Metal oxide semiconductor (MOS) transistors are building blocks for integrated circuits, and MOS devices continue to be scaled to smaller sizes for advanced performance targets. MOS technology includes N-type metal oxide semiconductor (NMOS) and P-type metal oxide semiconductor (PMOS) transistors.
An MOS transistor generally comprises a semiconductor substrate, a channel layer above the semiconductor substrate, a gate dielectric layer and a gate electrode above the channel layer, and source and drain diffusion regions in the surface of the semiconductor substrate. Contacts are made to the gate electrode and to both the source and drain regions of the transistor. With the advent of high-k dielectric materials as the gate insulating layer in the MOS process, metal gates or metal layers can be used in the devices.
A gate electrode comprises a conductive material and is electrically isolated from the semiconductor substrate by the gate dielectric layer. When a sufficiently high gate voltage is applied, an electrically conductive inversion layer or channel forms at the interface between the gate dielectric layer and the semiconductor substrate. The conductive channel extends between the source and the drain, whereby current flows through the channel when a voltage is applied between the source and drain.